A Transfer Matrix Model of the IEC 60318-4 Ear Simulator: Application to the Simulation of Earplug Insertion Loss

Abstract

The IEC 60318-4 ear simulator is used to measure the insertion loss (IL) of earplugs in the ear canal of an acoustical test fixture (ATF) and is designed to represent an average acoustic impedance (in a reference plane) of the human ear. The ear simulator is usually modeled using a lumped parameter model (LPM) which has frequency limitations and inadequately accounts for the thermo-viscous effects in the simulator. The simulator numerical models that can better deal with the thermo-viscous phenomena often lack essential geometric details. Most related studies also suffer from the lack of experimental validation of the models. Therefore, a transfer matrix (TM) model of the IEC 60318-4 simulator is proposed based on a direct assessment of its geometric dimensions. Such a model is of particular interest for designing artificial ear simulators. The variability in the simulator impedance due to the geometric uncertainties is quantified using the Monte Carlo method. The TM model is validated using i) a finite element (FE) model of the simulator and ii) impedance measurements with a sound intensity probe. It is found to better describe the simulator impedance above 3 kHz compared to the LPM. The TM model is then coupled to a FE model of an occluded ATF ear canal to simulate the IL of an earplug in the frequency range [100 Hz, 10 kHz]. In the model, the simulator is considered as a cylindrical cavity terminated by an equivalent tympanic impedance which is determined from the TM model to simulate the sound pressure measured at the real microphone position (not at the reference plane) in the ATF ear canal. The simulated IL is validated against i) that obtained with a complete FE model of the corresponding system and ii) measurements using an ATF. The TM model is shown to better agree with the simulator FE model than the LPM above 6 kHz regarding the earplug IL simulated using this method.

Reduction of the occlusion effect induced by earplugs using quasi perfect broadband absorption

“…5 a, the meta-earplug provides an insertion loss that has a curve similar to the silicone earplug. The curve of the silicone earplug compares well with literature data 28 , 29 . The local minimum of the silicone earplug insertion loss that occurs in the frequency band centered at 1.6 kHz comes from a piston-like mode of deformation of the system and depends on the apparent stiffness of the earplug compressed in the earcanal wall skin layer of the artificial test fixture 28 .…”

A finite element model to predict the double hearing protector effect on an in-house acoustic test fixture

“…Actually, the terminal surface of the inhouse ATF earcanal has been designed to correspond to the "eardrum position" in a commercial ATF earcanal where sound pressure is recorded. 16 More specifically, an internal miniature microphone together with a microphone holder is tightly attached to the ATF so that the microphone is flushmounted into the holder at the "eardrum position." A small cylindrical air cavity lies behind the microphone for passing its wire.…”